Lecture # 5 Modal or Dynamic Analysis of an Airplane Wing

Transcription

1 Lecture # 5 Modal or Dynamic Analysis of an Airplane Wing Problem Description This is a simple modal analysis of a wing of a model airplane. The wing is of uniform configuration along its length and its cross-sectional area is defined to be a straight line and a spline as shown. It is held fixed to the body of the airplane on one end and hangs freely at the other. The objective of the problem is to find the wing's natural frequencies and mode shapes. Given The dimensions of the wing are as shown above. The wing is made of low density polyethylene with a Young's modulus of 38x10 3 psi, Poisson's ration of 0.3, and a density of 1.033E-3 slugs/in 3. Approach and Assumptions Assume the side of the wing connected to the plane is completely fixed in all degrees of freedom. The wing is solid and material properties are constant and isotropic. Use solid modeling to generate a 2-D model of the cross-section of the wing, create a reasonable mesh and then extrude the cross-section into a 3-D solid model that will automatically be meshed. To minimize the solid modeling time, simplify the creation of the 2-D airfoil profile. To accurately follow the contour of this airfoil would require making more data points. Additionally, the mesh used in this example will be fairly coarse for the element types used. This coarse mesh is used here so that this tutorial can be used with the ANSYS/ED product. Summary of Steps Use the information in this description and the steps below as a guideline in solving the problem on your own. Or, use the detailed interactive step-by-step solution by choosing the link for step 1. Build Geometry 1. Create keypoints at given locations. 2. Create lines and splines between keypoints. 3. Create cross-sectional area. Define Materials 4. Define constant material properties. Generate Mesh 5. Set preferences. 6. Define element type. 7. Mesh the area. 8. Extrude the meshed area into a meshed volume.

2 Apply Loads 9. Unselect 2-D elements. 10. Apply constraints to the model. Obtain Solution 11. Specify analysis types and options. 12. Solve. Review Results 13. List the natural frequencies. 14. Animate the five mode shapes. 15. Exit the ANSYS program. Build Geometry 1. Create keypoints at given locations. -Modeling-Create Keypoints In Active CS 2. Enter 1 for keypoint number (ANSYS will automatically number keypoints if this field is left blank. These are generally preferred, but, for illustration, enter keypoint numbers manually). -» 3. Enter 0,0,0 for X,Y,Z location for keypoint 1. (Here, by leaving the fields blank, ANSYS will take their default value of zero). -» 4. Apply -» 5. Enter 2 for Keypoint number -» 6. Enter 2,0,0 for X,Y,Z location -» 7. Apply -» 8. Enter 3 for Keypoint number -» 9. Enter 2.3,0.2,0 for X,Y,Z location -» 10. Apply -» 11. Enter 4 for Keypoint number -» 12. Enter 1.9,0.45,0 for X,Y,Z location -» 13. Apply -» 14. Enter 5 for Keypoint number -» 15. Enter 1,0.25,0 for X,Y,Z location -» 16. OK -» 2. Create lines and splines between keypoints. -Modeling-Create -Lines-Lines Straight Line 2. Pick keypoints 1, 2, 5, 1 in this order (keypoint 1 is at the origin) 3. OK (in picking menu) Use keypoints 2 through 5 to draw a spline defining the curved part of the wing. ANSYS provides an option to define the orientation of an outward vector tangent at the first and last points on the spline. Use this option to define the spline so that it has a slope of zero at the bottom of the wing and a slope of 0.25 at the top, as shown in the problem sketch.

3 4. Main Menu: Preprocessor -Modeling-Create -Lines-Splines With Options Spline thru KPs 5. Pick keypoints 2, 3, 4, 5 in this order 6. OK (in picking menu) 7. Enter -1,0,0 for XV1, YV1, ZV1 -» 8. Enter -1,-.25,0 for XV6, YV6, ZV6 -» 9. OK -» 3. Create cross-sectional area. -Modeling-Create -Areas-Arbitrary By Lines 2. Pick all 3 lines 3. OK (in picking menu) 4. Toolbar: SAVE_DB Define Materials 4. Define constant material properties. Material Props -Constant-Isotropic 2. OK for material set number 1 -» 3. Enter for EX -» 4. Enter 1.033e-3 for DENS -» 5. Enter.3 for NUXY -» 6. OK -» Generate Mesh 5. Set Preferences 1. Main Menu: Preferences 2. Turn on Structural filtering -» 3. OK -» 6. Define element types. Define two element types: a 2-D element and a 3-D element. Mesh the wing cross-sectional area with 2- D elements, and then extrude the area to create a 3-D volume. The mesh will be "extruded" along with the geometry so 3-D elements will automatically be created in the volume. Element Type Add/Edit/Delete 2. Add -» 3. Structural solid family of elements -» 4. Apply to choose the Quad 4node (PLANE42) -» 5. Structural solid family of elements -» 6. Choose Brick 8node (SOLID45) -» 7. OK -» 8. Close -» 9. Toolbar: SAVE_DB

4 7. Mesh the area. The next step is to specify mesh controls in order to obtain a particular mesh density. Mesh Tool 2. Set global size controls -» 3. Enter 0.25 for element edge length -» 4. OK -» 5. Mesh -» 6. Pick All (in picking menu) 7. Close -» 8. Close Mesh Tool -» 9. Toolbar: SAVE_DB In designing this problem, the maximum node limit of ANSYS/ED was taken into consideration. That is why the 4-node PLANE42 element, rather than the 8-node PLANE82 element was used. Note that the mesh contains a PLANE42 triangle, which results in a warning. If you are not using ANSYS/ED, you may use PLANE82 during the element definitions to avoid this message. Note however that PLANE82 does not work unless you get rid of the Global Element edge length (which was set to 0.25). Note: The mesh you see on your screen may vary slightly from the mesh shown above. As a result of this, you may see slightly different results during postprocessing. 8. Extrude the meshed area into a meshed volume. In this step, the 3-D volume is generated by first changing the element type to SOLID45, which is defined as element type 2, and then extruding the area into a volume. -Modeling-Operate Extrude/Sweep Elem Ext Opt 2. Choose 2 (SOLID45) for Element type number -» 3. Enter 10 for the No. of element divisions -» 4. OK -» 5. Main Menu: Preprocessor -Modeling-Operate

5 Extrude/Sweep -Areas-By XYZ Offset 6. Pick All (in picking menu) 7. Enter 0,0,10 for Offsets for extrusion in the Z direction -» 8. OK -» 9. Close -» Using SOLID45 to run this problem in ANSYS/ED will produce this warning message. If ANSYS/ED is not being used, then SOLID95 (20-node brick) can be used as element type 2. Using PLANE82 and SOLID95 produces a warning message about shape warning limits for 10 out of 127 elements in the volume. 10. Utility Menu: PlotCtrls Pan, Zoom, Rotate 11. Choose ISO -» 12. Close -» Apply Loads 9. Unselect 2-D elements. Before applying constraints to the fixed end of the wing, unselect all PLANE42 elements used in the 2-D area mesh since they will not be used for the analysis. 1. Utility Menu: Select Entities 2. Choose Elements -» 3. Choose By Attributes -» 4. Choose Elem type num -» 5. Enter 1 for the element type number -» 6. Choose Unselect -» 7. Apply -» 10. Apply constraints to the model. Constraints will be applied to all nodes located where the wing is fixed to the body. Select all nodes at z=0, then apply the displacement constraints. 1. Choose Nodes -» 2. Choose By Location -» 3. Choose Z coordinates -» 4. Enter 0 for the Z coordinate location -» 5. Choose From Full -» 6. Apply -» 7. Main Menu: Preprocessor Loads -Loads- Apply -Structural-Displacement On Nodes 8. Pick All (in picking menu) to pick all selected nodes 9. Choose All DOF -» 10. OK (Note: By leaving Displacement value blank, a default value of zero is used.) -» Reselect all nodes. 11. Choose By Num/Pick -» 12. Sele All to immediately select all nodes from entire database -» 13. Cancel to close dialog box -»

6 14. Toolbar: SAVE_DB Obtain Solution 11. Specify analysis type and options. Specify a modal analysis type. 1. Main Menu: Solution -Analysis Type-New Analysis 2. Choose Modal -» 3. OK -» 4. Main Menu: Solution -Analysis Type-Analysis Options 5. Choose Block Lanczos -» 6. Enter 5 for the No. of modes to extract -» 7. Enter 5 for the No. of modes to expand -» 8. OK -» 9. OK (All default values are acceptable for this analysis.) -» 10. Toolbar: SAVE_DB 12. Solve. 1. Main Menu: Solution -Solve-Current LS 2. Review the information in the status window, then choose File Close (Windows NT / Windows 95) 3. OK to initiate the solution -» 4. Yes -» 5. Yes -» Based on previous discussions, the warnings are accepted. The messages presented in the verification window are due to the face that PLANE42 elements have been defined but not used in the analysis. They were used to mesh a 2-D cross-sectional area. 6. Close to acknowledge that the solution is done -» Review Results 13. List the natural frequencies. 1. Main Menu: General Postproc Results Summary 2. File Close (Windows NT / Windows 95)-» 14. Animate the five mode shapes. Set the results for the first mode to be animated. 1. Main Menu: General Postproc -Read Results-First Set 2. Utility Menu: PlotCtrls Animate Mode Shape 3. OK -» For Windows NT systems only: the Media Player appears. Make sure the Auto Repeat option is selected before

7 viewing the animation. Animate the next mode shape. 4. Main Menu: General Postproc -Read Results-Next Set 5. Utility Menu: PlotCtrls Animate Mode Shape 6. OK -» Observe the second mode shape: Observe the third mode shape: Observe the fifth mode shape:

8